The present invention relates to a three port structure for the down conversion of a modulated RF signal, a RF receiver comprising such a three port structure, a mobile telecommunications device comprising such a three port structure as well as to a method for the down conversion of a modulated RF signal by means of a three port structure.
A three port structure is a particular case of n-port structures. In the following the state of the art particularly regarding six-port structures (n=6) will be explained.
A six-port receiver is known acting in a direct conversion manner and allowing conversion from mm-wave range and microwave range directly to the base band. The six-port receiver detects the relative phase and relative magnitude of two incoming RF-signals by using the information of superimposed RF signals. At the same time a classic I/Q-demodulation chip (digital or analog) can be avoided. By using suitable calibration procedures the influences of the non-ideal linear RF-components including manufacturing tolerances can be minimized. The circuitry of the six-port receiver is realized using only passive components in combination with power sensors for the detection of the relative phase and the relative magnitude of the RF-signals as shown in EP-A-0896455.
In Bossisio, Wu xe2x80x9cA six-port direct digital millimeter wave receiverxe2x80x9d, Digest of 1994 IEEE MTT Symposium, vol. 3, page 1659-1662, San Diego, May 1994, a structure for a six-port receiver is proposed.
The six-port technique has been known for its ability to accurately measure the scattering parameters, both amplitude and phase, of microwave networks. Instead of using heterodyne receivers a six-port receiver accomplishes direct measurements at microwave and mm-wave frequencies by extracting power levels at at least three different ports. The imperfections of the hardware can be readily eliminated by an appropriate calibration procedure. Very accurate measurements can be made in a large dynamic range and wide frequency range. Six-port junction receivers consist of microwave components such as e.g. directional couplers and power dividers as well as power sensors. The circuit can be easily integrated as MHMIC or MMIC. The known receiver performs direct phase/amplitude demodulation at microwave and mm-wave frequencies.
By performing a calibration procedure the hardware imperfections can be readily eliminated. This significantly eases the requirement of the hardware implementation and enables the six-port receiver to operate over a wide band up to mm-wave frequencies.
According to the above cited document of Bossisio et. al. a six-port receiver concept with power dividers and 90 degrees hybrid circuits realized in distributed technology is used. The application of that known structure lies mainly in the frequency bands above 10 GHz, however, it suffers from an insufficient band width of the operation due to the inherently frequency selective nature of the 90 degree hybrid circuits.
From D. Maurin, Y. Xu, B. Huyart, K. Wu, M. Cuhaci, R. Bossisio xe2x80x9cCPW Millimeter-Wave Six-Port Reflectometers using MHMIC and MMIC technologiesxe2x80x9d, European Microwave Conference 1994, pp. 911-915, a wide-band topology for reflectometer used is known which is based on a distributing element approach featuring coplanar wave guide applications in the frequency range from 11 to 25 GHz.
From V. Bilik, et al. xe2x80x9cA new extremely wideband lumped six-port reflectometerxe2x80x9d European Microwave Conference 1991, pp. 1473-1477 and the idea of using Wheatstone Bridges and resistive structures for reflectometer applications is known.
From Li, G. Bossisio, K. Wu, xe2x80x9cDual tone Calibration of Six-Port Junction and its application to the six-port direct digital receiverxe2x80x9d, IEEE Transactions on Microwave Theory and Techniques, vol. 40, January 1996 a six-port reflectometer topology based on four 3dB hybrid circuits, power dividers and attenuators is known.
From U.S. Pat. No. 5,498,969 an asymmetrical topology for a reflectometer structure featuring one matched detector and three unmatched detectors is known.
From U.S. Pat. No. 4,521,728 with the title xe2x80x9cMethod and six-port network for use in determining complex reflection coefficients of microwave networksxe2x80x9d a reflectometer six-port topology is known comprising two different quadrate hybrids, phase shifter, two power dividers and one directional coupler for which the realization by a microstrip line technology is disclosed.
From EP-A-0 805 561 a method for implementing a direct conversion receiver with a six-port junction is known. According to this known technique, modulated transmitted modulation is received by a direct conversion receiver which comprises a six-port junction. The demodulation is carried out analogically.
From EP-A-0 841 756 a correlator circuit for a six-port receiver is known. In this correlator circuit the received signal is summed up with a local oscillator signal at various phase angles, wherein the phase rotation between the local oscillator and RF signals is carried out separately from the summing of the correlator outputs.
A three port structure has particularly advantages due to its less complexity.
It is the object of the present invention to provide for a technique for further simplifying three port structures. Particularly the technique according to the invention should give the possibility to suppress any RF switches in the three port structure.
The object is achieved by means of the features of the independent claims. The dependent claims develop further the central idea of the present invention.
According to the invention therefore a three port structure for the down conversion of a modulated RF signal is provided. The three port structure has a first input being supplied with the RF signal to be down converted. A second input is provided for a RF signal originating from a local oscillator. An output of the three port structure supplies a signal representing a combination of the RF signals supplied to the first and second input to a power sensor unit. Between the local oscillator and the three port structure a modulator is provided such that the RF signal from the local oscillator is modulated according to a predetermined modulation scheme before it is supplied to the second input of the three port structure.
The modulator can be particularly designed to generate at least three different modulation states of a predetermined modulation scheme within the time duration of one modulation state of the RF signal to be down converted such that three different combination signals are output to the power sensor unit within the time duration of one modulation state of the RF signal to be down converted.
A processing unit can detect modulation states of the modulated RF signal supplied to the first input of the three port structure respectively on the basis of three different combination signals output by the three port structure during the time duration of one modulation state of the RF signal supplied to the first input of the three port structure.
The modulator can be designed to generate modulation states with at least two different amplitudes and phases.
The modulator can be designed to generate modulation states with equal amplitude and at least three different phase states (PSK modulation scheme).
A (non-reciprocal) RF/LO isolation unit can be provided in the three port structure for isolating the first and the second input port of the three port structure.
The power sensor unit can comprise a plurality of power sensors, wherein at least some of the power sensors have a power coupler at its input side having a coupling coefficient less than the dynamic range of the associated power sensor.
According to another aspect of the present invention a RF receiver comprising a three port structure as set forth above is provided.
According to a still further aspect of the present invention a mobile telecommunications device such as a mobile telephone is provided, wherein the mobile telecommunications device comprises a three port structure as set forth above.
According to a still further aspect of the present invention a method for the down conversion of a modulated RF signal by means of a three port structure is provided. A RF signal to be down converted is supplied to a first input of the three port structure. A RF signal originating from a local oscillator is supplied to a second input of the three port structure. The RF signals supplied to the first and second input are combined in the three port structure and a signal representing the result of the combination is output to a power sensor unit. The RF signal from the local oscillator thereby is modulated before it is supplied to the second input of the three port structure.
In the modulation step at least three different modulation states are generated according to a predetermined modulation scheme within the time duration of one modulation state of the RF signal to be down converted. Thereby three different combination signals are output to the power sensor unit within the time duration of one modulation state of the RF signal to be down converted.
The modulation states of the modulation RF signal supplied to the first input of the three port structure can be detected respectively on the basis of at least three different combination signals output by the three port structure during the time duration of one modulation state of the RF signal supplied to the first input of the three port structure.
In the modulation step modulation states with at least two different amplitudes and phases can be generated.
Alternatively, in the modulation step modulation steps with equal amplitude and at least three different phase states can be generated (PSK modulation scheme).
The first and the second input of the three port structure can be isolated to avoid a coupling from the RF signal supplied to the second input of the three port structure to the signal supplied to the first input.